PFR lime kilns are used for burning limestone and comprise according to FIG. 1 at least two shafts 1, 2 which each have a preheating zone V, a combustion zone B and a cooling zone K. Both shafts are connected to each other by means of an overflow channel 3. The material to be burnt is introduced from above into both shafts and removed at the bottom as burnt material.
Both shafts are operated alternately as a combustion shaft and an exhaust gas shaft, combustion air being supplied to the combustion shaft in direct current with the material and fuel and the resultant hot exhaust gases being directed into the exhaust gas shaft via the overflow channel together with the heated cooling air which is supplied from below, where the exhaust gases are directed away upwards in counter-current to the material and preheat the material. After a predetermined period of time, for example, of 15 minutes, the function of the two shafts is interchanged, that is to say, the combustion shaft becomes the exhaust gas shaft and vice versa. This method allows very efficient burning of the limestone in direct current with the combustion gases and a regenerative preheating of the limestone in counter-current to the hot exhaust gases.
Owing to various disturbance variables, such as, for example, fluctuations in the calorific value of the fuel, fluctuations in the carbonate content of the raw material or fluctuations of the heat losses of the kiln, a readjustment of the quantity of fuel may be required. Another disturbance variable is formed by kerogen components which are often contained in the limestone which is to be burnt. This is an organic polymer material which releases hydrocarbons during heating. However, these kerogens are not distributed in a homogeneous manner in the raw material so that readjustment of the specific heat supply of the lime kiln of up to 6% is required in order to achieve a constant product quality.